Television system



Dec 14, 1937. R. l.. CAMPBELL TELEVISION SYSTEM Original Filed May 26, 1933 I I I l I l l I l l l l n I l I l l I J 12d L Camlwll,

/Is ZITTOR/VEK Patented Dec. 14, 1937 NETED STATES TELEVISION SYSTEM Richard L. Campbell, Oaklyn, N. J., assignorto Radio Corporation of America, a corporation of Delaware Application May 26, 1933, Serial No. 672,963

Renewed October 16, 1936 14 Claims.

lVly invention relates to improvements in television systems.

In a television transmission system of oneV type, the picture signals are developed by apparatus in the form of a cathode ray tube comprisinge. mosaic photosensitive screen ontol which an image ofthe object is projected, and an electron gun which develops a ray of electrons and directs the same at the screen. In operation, the ray is caused to scan the screen by electromagnetic coils or by plates. When coils are used, a saw-tooth current wave at a relatively high frequency is caused to pass through one set of coils to deflect the ray horizontally, and a sawtooth current wave at a relatively low frequency is caused to pass through a second set of coils to deflect the ray vertically.

In these systems, it is required that synchronizing impulses at the relatively high frequency and framing impulses at the relatively low frequency be developed at the transmitter and transmitted with the picture signals for util- V ization at the receiving stations for the purpose of maintaining operating action thereat in synchronism and in frame with operating action at the transmitting station.

In the systems of the prior art, the synchronizing and framing impulses referred to have been developed by employingV some form of mechanical device such as a disc rotated at a constant speed and provided with apertures suitably spaced and of the required shapes for controlling admission of a beam of light to a photoelectric cell. These impulses have then been amplified and mixed directly with the picture signals for transmission. In this Way, control impulses of the proper shape, time and amplitude have been obtained.

One of the objects. of my invention is to pro- 40 vide an improved television transmission system of the character referred to wherein the synchronizing and framing impulses of the proper shape, timing and amplitude are developed without recourse to the use of a mechanu ical device at the source of development of these impulses, and also to provide means for controlling the duration and amplitude of these impulses without requiring mechanical changes.

Other objects and advantages will hereinafter appear.

In accordance with my invention, the horizontal and vertical return-line voltages across the two sets of deflecting coils referred to are applied to Vspecial impulse amplifier circuits which pro- 55 duce square-topped impulses occurring at the time of the horizontal and vertical return-line periods during scanning action of the cathode ray.

My invention resides in the improved system, apparatus and method of the character herein- 5 after described and claimed.

For the purpose of illustrating my invention, an embodiment thereof is shown in the drawing, wherein the ligure is a diagrammatic View of a television transmission system embodying and operating in accordance with my invention.

The reference numeral IIJ designates a cathode ray tube for developing picture signals. This tube comprises suitable photosensitive, mosaic screen structure I2 which, for example, may be made up of a great number of minute, photosensitized silver globules supported on a metallic base plate and each insulated from the latter and from each other. A suitable electron gun I4 operates to develop a ray I6 of electrons and to direct the same at the screen structure. The tube is provided with a grounded anode in the form of a silver coating Il on the inside surface.

'Ihe ray I6 is caused to scan a rectangular area or frame of the screen structure twenty-four times a second. During relatively short periods between the frames, the ray is returned to the initial starting point for the next succeeding frame.

Let it be assumed, for example, that there are to be 24 pictures or frames a second, each having 180 lines horizontally. For these conditions, the rayA is deflected horizontally by coils I8 through which a saw-tooth current wave is caused to pass at a frequency of 4320 cycles. A` suitable generator 2D supplies the current wave for this purpose. Also, the ray is deflected vertically by coils 22 Vthrough which a saw-tooth current wave is caused to pass at a frequency of 24 cycles. A suitable generator 24 supplies the current wave for this purpose.

The electron gun I4 is provided with a control electrodeV or grid 26 by which the intensity of the ray is varied in the manner and for the purpose hereinafter explained. 45

An image of the object 28, which may be an outdoor view, a studio scene, or a moving-picture film, is projected onto the screen I2 by a suitable lens system 30. y

Looking toward the right in the drawing, as the ray I6 scans the screen from left'to right, individual electrostatic charges, previously stored on the globules constituting the surface of the screen are neutralized to develop picture signals lin the output circuit of the tube I0. These signals are amplified in a suitable amplifier 46 and supplied by a connection 42 to a radio transmitter 44.

For the purpose of maintaining operating action of the generators 2@ and 24 at the respective frequencies, .a suitable generator 46 is utilized to develop control signals which occur at the rate of 4320 a second, and other control signals which occur at the rate of 24 a second. The relative amplitude of these control signals and their occurence with respect to the time axis is indicated at 48 and 5t, respectively, from which it will be noted that the control signals for vertical deflection are of such longer duration than the control signals for horizontal deflection, and that both of these signals are of the same amplitude. To avoid prolixity, the signals first referred to will, hereinafter, be designated as horizontal-control signals, and the second mentioned signals as vertical-control signals.

The generator 4E may be in the form of an apertured disc of the 'general type disclosed in the copending application by Arthur W. Vance, Serial No.v 544,959, filed July 17, 1931 and assigned to Radio Corporation of America. The apertures in the disc (not Shown) are such that the control signals have the same amplitude, the verticalcontrol signals, however, being of greater duration than the horizontal-control signals and occupying a time period equal to three or four horizontal return-line periods. After amplification in an amplifier 52, the control signals are impressed across resistors 54 and 56 and applied therefrom to the generators 20 and 24, respectively.

The construction and adjustments of the generator 24 are such .that the same is responsive only to the Vertical-control signals of substantially greater duration than. the horizontal-control signals.

As indicated in the drawing, the tube l is supplied with the usual operating potentials.

With reference now to the novel features in which my invention resides, a circuit A is utilized for the purpose of developing thesynchronizing and framing impulses for transmission. This circuit comprises an amplifier tube 58 having its grid circuit connected as shown tothe plate circuit of .an ampliiier tube Gil, and an amplifier tube 62 having its grid circuit connected to the plate circuit of an amplifier tube 64. The plates of the ytubes 58 and 62 have a common resistor 66 for the purpose hereinafter explained. The voltage Wave appearing across the horizontal deiiecting coils I8V includes a saw-tooth component and an impulse component, and is applied by a connection l2 from these coils to the grid of the tube 6D. The relative amplitudes and shapes of these components and their occurence with respect to the time axis is indicated at 68 Vand l0, respectively. Hereinafter, the impulse components will be referred to as synchronizing impulses. f

The voltage wave appearing across the vertical deflecting coils 22 also comprises a saw-tooth component and an impulse component, and is applied by a connection 'i8 from these coils to the grid of the tube 64. The relative amplitudes and shapes of these impulses and their occurrence With respect to the time axis is indicated at 1,4 and '16, respectively. Hereinafter, the impulse components will be referred to as framing impulses.

The vsynchronizing impulses occur at the required frequency and are of the proper shape, amplitude and duration for synchronizing purposes. Also, theframing'impulses occur at the required frequency and are of the proper shape and amplitude for framing purposes. The synchronizing impulses on the grid circuit of the tube 60 and the framing impulses on the grid circuit of the tube 64 are of positive polarity for the reason that the return-line voltages across the coils I8 and 22 are cf positive polarity.Y This is because the most eiiicient operation of the output tubesV in the generators 2S and 24 which drive the coils I8 andVZZ, respectively, is obtained when the impulses on their grids during these periods are of negative polarity.

The tubes 65 and 64 are biased negatively beyond plate-current cut-ori, and the return-line scanning voltages are of the order of hundreds of volts, While the-picture scanning voltages represented by the saw-tooth component are below 10% of the value of the return-linevoltages. Also, the picture voltages are negative, while the return-line voltages are positive. For these reasons, the synchronizing and framing impulses on the plates of the tubes @t and 64, respectively, are in the form of negative impulses, as indicated Vat 36 and 82, and .are of amplitude corresponding to plate saturation of the respective tubes. Also, the framing impulses, occurring at the rate of 2d a second, are of greater duration than the synchronizing impulses which occur .at the rate ci 4320 a second, by an amount corresponding to the difference between the duration of the horizontal control signals and the vertical control signals. Since both tubest!! and 64 are biased beyond cut-off so that only the positive returnline voltages are effective to cause them to draw plate current, and to drive them to plate current saturation, the impulses in their plate circuits are flat-topped. Y

The tube 58 operates to amplify the synchronizing impulses, which are of positive polarity in the plate Circuit of this tube, as indicated at 34. The tube 62 operates to amplify the framing impulses, which are of positive polarity in the plate circuit of this tube, as indicated at 86.

The synchronizing and framing impulses are mixed in the resistor 66, and are supplied by a connection 88 to an amplier circuit B.

The synchronizing and framing impulses, as amplified in the circuit B, are then used for three distinct purposes, as follows: (a) by means of a connection 90, these impulses are taken from a suitable stage of the circuit B, whereat the same are of suflicient amplitude and of negative polarity, as indicated by the figure 62, and are applied to the grid 26 for the purpose of cutting off the ray I6 during the horizontal and vertical returnline periods; (b) by means of a connection 94, the synchronizing and framing impulses are'taken from a suitable stage of the circuit E, whereat the same are of sufficient amplitude and of negative polarity, as indicated by the ligure S6, and are applied to a selected stage of the amplifier 4i) whereat these impulses are effective to .block out this amplifier during the horizontal and vertical return-line periods. In this manner, the amplier 4Q is prevented from passing and amplifying undesirable signals which might be developed during these return periods; and (c) by means of a connection 98, the synchronizing framing impulses are taken from a suitable stage. of the circuit B, Whereat they are of sunicient amplitude and of positive polarity, as indicated by the figure |66, and are applied to a stage of the amplifier 40 which is beyond the stage at which the connection 94 is made, as indicated inthe drawing, the actionbeing to cause the synchronizing and framing impulses -to be amplied and transmitted with the picture signals from this point on. The amplitude of the synchronizing and framing impulses in the connection 98 is such that they drive the tube supplied by this connection beyond the point of platecurrent saturation. In this manner, and by making the connections 94 and 98 as explained, the synchronizing and framing impulses transmitted with the picture signals are flat-topped, and are accompanied by practically no extraneous signals which otherwise might operate at the receiving stations to interfere with proper synchronizing action.

In operation, the amplitude of the framing impulses is adjusted until they are substantially equal to the synchronizing impulses. This is accomplished by adjusting the potentiometer i532 in the plate circuit of the tube 64.k As stated, the synchronizing and framing impulses are negative on the grids of the tubes 58 and 62 which are cut off by these impulses. Any impulses of positive polarity are blocked or prevented from passing by these tubes because no negative bias has been provided therefor, as represented in the drawing, and the grids of these tubes are prevented from swinging positive due to grid current in the high resistance shown connected in the grid circuit. Y

The width of the synchronizing impulses is affected and can be varied by changing the value of a resistance |04 connected in series with the grid of the tube 58. An increase in the value of this resistance operates to increase the Width or duration of these impulses. The Width of the framing impulses can beincreased by inserting a relatively small capacity |06 from the grid of the tube 62 to ground. The Widths of the synchronizing and framing impulse-s can also be changed by adjusting the bias voltages on the first impulse tubes B0 and 64. Y l

While it is contemplated to supply the tubes and 64 directly from the deflecting coils i8 and 22, or from the deflecting plates when these are used, it Will be understood that the connections l2 and 18 can be made from other points in the respective generators 20 and 24 which provide the impulses having the proper amplitudes, durations and time occurrence for derivation of the synchronizing and framing impulses. In such case, the connections 'I2 and 18 shown are omitted, and the connections made as indicated by the broken lines 12a and 78a.

From the foregoing it will be seen that I have provided an improved television transmission system wherein the synchronizing and framing impulses are developed for transmission without c recourse, at the source of development of these impulses, to the use of a mechanical device such as an apertured disc, thereby not only eliminating the disc at this point in the system, but also the associated parts required heretofore such as a light source, a photocell and a photocell amplifier. With the omission of the disc, the undesirable eiects of dust particles accumulating in the disc apertures are eliminated. Other advantages in my improved system reside inthe fact that the accuracy of the synchronizing and framing impulses is certain, since they are derived directly from the deflecting circuits and, therefore, must always occur during the horizontal and vertical return-line periods. This desirable actiony is manifested at the receiving stations by a substantial improvement in possible detail of the reproduced picture.

While but one embodiment of my invention has been shown and described, it'will be understood that various changes Within the conception of those skilled in the art can be made'without i departing from the spirit of my invention or the scope of the claims.

I claim as my invention: Y

1. In a television transmission system, electron apparatus for develop-ing picture signals, means for developing control signals, means for eecting scanning action of said' apparatus and including a generator of an electrical Wave of a given irequency, said generator being supplied with said control signals and maintained in operation thereby at said .given frequency, and means supplied With electrical impulses from said generator and operating to develop therefrom synchronizing impulses ior transmission.

2. In a television transmission system, electron apparatus for developing picture signals, means for developing control signals, complementary elements disposed respectively on opposite sides of the longitudinal axis of said apparatus for effecting deflection of the electrons, means for generating an electrical Wave of a given frequency and operating to impress the same across said elements, said generato-r means being supplied With said signals and maintained in operationy thereby at said given frequency, and a circuit vhaving its input lines connectedgacross said ele- Vments and operating to develop synchronizing apparatus for developing picture signals, means for effecting deiiection of the electrons including a generator of an electrical wave comprising a saw-tooth component and an impulse component, an electron tube upon the input circuit of which said Wave is impressed under conditions Whereat the impulse components are of positive polarity, said tube operating to pass only the impulse components, a second electron tube supplied from the iirst tube and operating to reverse the polarity of the impulse components, and

means supplied from said second tube'and oper- Y ating to amplify said impulse components f r transmission.

5. In a television transmission system, electron apparatus for developing picture signals, means for effecting deilection of the electrons and including generators of electrical Waves at relatively high and relatively low frequencies respectively. means upon which said Waves are impressed and operating to develop therefrom synchronizing nals occurring at the relatively high frequency andI framing signals occurring at the relatively low frequency, a multi-stage amplier for amplifying the picture signals, a connection supplying the synchronizing and framing signals to a stage of said amplier and at the required polarity Vto block the latter against amplifying action, and a second connection supplying the synchronizing and framing signals to a stage of said amplifier between said first-named stage and the output circuit of said amplifier and at the required polarity whereby the synchronizing and framing signals are amplified with the picture signals for transmission. Y i

6. In a television transmission system, electron apparatus for developing picture signals, means for effecting deectionof the electronsV and including generators of electrical Waves at relatively high and relatively low frequencies respectively, each wave comprising a saw-tooth component and an impulse component, means upon which is impressed the Wave at the relatively high frequency and operating to pass only the impulse components of this Wave to the exclusion of the saw-tooth components and to develop synchronizing signals at the relatively high frequency, means upon which is impressed the wave at the relatively low frequency and operating to pass only the impulse components of this wave to the exclusion of the saw-tooth components and to develop framing signals at the relatively low frequency, and means for transmitting the picture signals and the synchronizing and framing signals.

7. In a television transmission system, apparatus for developing picture signals, a source of electrical impulses at a relatively high synchronizing frequency, a source of electrical impulses at a relatively low framing frequency, an electron tube to the input circuit of which the synchronizing impulses only are applied, a second electron tube to the input circuit of which the framing irnpulses only are applied, a resistor common to and associated with the output circuits of said tubes and across which both the synchronizing and framing impulses appear at the same polarity, and amplifying means supplied from said resistor and operating to amplify both the synchronizing and framing impulses for transmission.

8. In a television transmission system, apparatus for developing picture signals, a source of electrical impulses at a relatively nigh synchronizing frequency and of positive polarity, a source of electrical impulses at a relatively low framing frequency and of positive polarity, an electron tube to the input circuit of which the synchronizing impulses only are applied and which operates to reverse the polarity thereof to negative, a second electron tube to the input circuit of which the framing impulses only are applied and which operates to reverse the polarity thereof to negative, means including a resistor for combining the synchronizing and framing impulses to cause the same to appear across said resistor at the same polarity, and amplifying means supplied from said resistor and operating to amplify both the synchronizing and framing impulses for transmission. f

9. In a television transmission system, electron apparatus for developing picture signals, electromagnetic coils for deflecting the electrons, a generator of a saw-tooth current wave at a given frequency and operating to cause such wave to pass through said coils, means supplied from said generator and operating to develop impulses occurring at said frequency, and means supplied with the picture signals and with said impulses and operating to amplify the same together for transmission.

10. In a television transmission system, electron apparatus for developing picture signals, means for effecting scanning action of said apparatus and including a generator oi an electrical wave at a relatively high synchronizing irequency and a generator of an electrical Wave at a relatively low framing frequency, means supplied from said first-named generator and operating to develop impulses at the synchronizing irequency, means supplied from said second-named generator and operating to develop impulses at the framing frequency, and meansA common ,to and supplied from both of said impulse-developing means and operating to 'amplify the synchronizing and framing impulses for transmission.

11.7111 a television transmission system, apparatus for developing picture signals, a source of electrical impulses at a relatively high synchronizing frequency, a source of electrical impulses at a relatively low framing frequency, a circuit to which the synchronizing impulses only are applied, a circuit to which the framing impulses only are applied, a resistor common with respect to said circuits and across which both the synchronizing and framing impulses appear at the same polarity, and amplifying means supplied from said resistor and operating toY amplify both the synchronizing and framing impulses for transmission.

12. In a television transmission system, scanning apparatus forV developing picture signals, a multi-stage amplier supplied with said signals,

means for developing Vcontrol signals at a ,fre-V quency corresponding to the scanning frequency, a connection supplying control signals at said frequency to a stage of said amplifier and at the required polarity to block said amplifier against amplifying action, and a connection supplying control signals at said frequency to a stage of said amplifier between said rst-named stage and the output circuit of said amplifier and at the required polarity whereby the control signals at said frequency are amplied with-the picture signals for transmission. Y y

13. In a transmitter for the transmission of picture signals, a cathode ray tube, electromagnetic coils for deflecting the cathode ray, means for producing a flow of dellecting current through Y said coils, said defiecting current having periodically recurring portions of saw-tooth wave shape whereby voltage impulses are produced at the frequency at which said portions recur, and means for supplying said impulses and said picture signals to a common transmission channel whereby said impulses are transmitted as synchronizing impulses. i

14. In a transmitter for the transmission of picture signals, a'cathoderay tube, eiectromagnetic coils for deiiecting the cathode means for producing a flow of deiiecting current through said coils, said deilecting current ing periodiwhereby said impulses are transmitted as synchrcnizing impulses.

RICHARD L. CAMPBELL. 

